1. Field of the Invention
The present invention relates to an image forming apparatus of the electrophotographic type such as copying apparatus, printers and the like.
2. Description of the Related Art
Conventional image forming apparatus of the electrophotographic type typically use two-component developing devices which develop an electrostatic latent image formed on the surface of a latent image carrier by means of a two-component developer comprising a toner and a carrier. In this type of developing device, the toner and carrier must be mixed by means of a stirring member so as to electrically charge the toner. Accordingly, the two-component developer accommodated within a two-component developing device is generally mixed for a uniform time prior to the operation of the developing member so as to electrically charge the toner with a charge greater than a predetermined value. (This operation is referred to as "pre-development mixing" hereinafter.) When the operation of the developing member starts, The mixing member mixes the two-component developer within the developing device, or supplied developer to the developing member while mixing the two-component developer accommodated within the developing device and mixing the toner resupplied from the toner hopper into the developing device. The two-component developer supplied to the developing member is used to develop the electrostatic latent image formed on the surface of the latent image carrier.
The minimum time required for the aforesaid pre-development mixing (this time is referred to as "mixing time" hereinafter) varies depending on the condition of the aforesaid developer. The condition of the developer is determined by the toner density of the two-component developer accommodated in the developing device when mixing is started, the time during which the two-component developer has been left standing idle, frequency of use of the two-component developer and the like. The influence of the developer condition on the mixing time is shown in FIGS. 1-3. As is clearly shown in the aforesaid drawings, developer which has, for example a high toner density, stood idle over a long period, and deteriorated over long-term use requires a long mixing time to achieve a toner charge amount equal to or greater than a predetermined value Q.sub.1. If the developing member is operated while the amount of charge is not Q.sub.1 or greater, background fog and toner dispersion occur due to inadequate toner charging. Therefore, the mixing time is conventionally set at a uniform time so as to be of a length to allow sufficient charging of the toner even under the previously mentioned conditions of the developer.
In the previously described developing device, excessive mixing of the developer occurs when a long mixing time is not required, thereby accelerating the deterioration of the developer. Furthermore, the toner becomes excessively charged due to over mixing, such that the amount of toner adhering to the latent image carrier is reduced, thereby producing a decrease in image density.
U.S. Pat. No. 4,338,019 discloses a developing device for minimizing the mixing time to obtain a predetermined amount of toner charge, wherein the developer is mixed while the amount of toner charge is detected based on the output of a toner density sensor so as to operate the developing member when the toner charge reaches a predetermined level. However, in the case of this device, the amount of charge of the entire developer cannot be accurately detected because the amount of charge of the developer in the vicinity of the toner density detection area is unstable during the initial mixing period. Accordingly, mixing is stopped before the amount of charge of the entire developer is stabilized, resulting in mixing in excess of requirements.